From http://sourceforge.net/p/pdl/bugs/391/

The docs for many functions in PDL::Ops give 3 ways to call the function. Using 'lt' as an example:

   $c = lt $a, $b, 0;     # explicit call with trailing 0
   $c = $a < $b;           # overloaded call
   $a->inplace->lt($b,0);  # modify $a inplace

The first form does not work as advertised.

$a = pdl(1,2,3); $b = pdl(3,2,1);
$c = lt $a, $b, 0;
syntax error at (eval 67) line 4, near "= lt"

Similarly 'plus' gives this example that does not work:

$c = plus $a, $b, 0;
Usage:  PDL::plus(a,b,c,swap) (you may leave temporaries or output variables out of list)

and so on... The ->inplace form seems to work OK.

perldl -V attached.

Marking as priority '7' because it seems pretty important, but my guess is very few people actually try to call the functions that way (as evidenced by the fact that nobody has reported this before).